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Fracture Identification and Evaluation Using S Waves; #40792 (2011)

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Fracture Identification and Evaluation Using S Waves; #40792 (2011) GCFracture Identification and Evaluation Using S Waves* Bob Hardage1 Search and Discovery Article #40792 (2011) Posted August 15, 2011 *Adapted from the Geophysical Corner column, prepared by the author, in AAPG April-August Explorers, 2011, and entitled, respectively “S Waves Prove Their Worth With Fractures”, “Turn, Turn, Turn: Rotating for S-Wave Data”, “S-Waves on Crack? Not So Much”, “Measuring Fractures – Quality and Quantity”, “For Fractures, P + S = Maximum Efficiency”. Editor of Geophysical Corner is Bob A. Hardage ([email protected]). Managing Editor of AAPG Explorer is Vern Stefanic; Larry Nation is Communications Director. 1Bureau of Economic Geology, The University of Texas at Austin ([email protected]) General Statement When a shear (S) wave propagates through a rock unit that has vertical fractures oriented at a reasonably consistent azimuth, it splits into two S waves that propagate with distinct velocities. ● One of these S waves is a fast-velocity mode called S1, which is polarized in the same direction as the fracture orientation. ● The other is a slow-velocity mode called S2, which is polarized in a direction orthogonal to the fracture planes. The S1 mode has approximately the same velocity as an S wave that propagates in the rock when fractures are absent. In contrast to this S- wave physics, a compressional (P) wave does not split into fast and slow modes when it encounters a fractured interval. When fracturing causes significant differences in elastic moduli parallel and perpendicular to fractures, P-wave velocity can vary when measured parallel to and perpendicular to oriented fractures, as does S-wave velocity – but differences in P-wave velocity are not as dramatic as those in S-wave velocity. Thus, S waves are preferred over P waves for seismic-based evaluations of fractured rocks. S-Wave Splitting Phenomenon S-wave splitting phenomenon is illustrated on Figure 1, where an S wave illuminates a zone of well-aligned vertical fractures. The incident S wave is polarized so that its particle-displacement vector is oriented at an angle Ф relative to the azimuth of the vertical fractures. S1 and S2 Copyright © AAPG. Serial rights given by author. For all other rights contact author directly. modes exit the base of the fracture zone at different times because they propagate with different velocities inside the fracture space (S1 = fast; S2 = slow). As expected, the S1 mode is polarized parallel to the fracture planes, and the S2 mode is polarized perpendicular to the fracture planes. S1 and S2 modes also reflect from the fracture zone, but are not shown. A laboratory test that documents the S-wave physics described by this model was published by Sondergeld and Rai (1992). Their test procedure is illustrated on Figure 2. In this test, a piezoceramic element was secured to one end of a cylindrical volume of laminated shale to serve as an S-wave source. A similar piezoceramic element was positioned at the opposite end of the cylinder as an S-wave sensor. This layered propagation medium, and the fact that the source-receiver geometry causes S-waves to propagate parallel to the embedded interfaces of the rock sample, are a good simulation of S-wave propagation through a system of vertical fractures. In one test, the source remained in a fixed orientation relative to the plane of the simulated fractures and the receiver element was rotated at azimuth increments of 10 degrees to determine the azimuth dependence of S-wave propagation through the sample. The test results are illustrated on Figure 3 as an end-on view of the test sample from the source end; the objective was to simulate the propagation of a fast-S (or S1) mode, where the source displacement vector is parallel to the fracture planes (Figure 3a), and then to simulate the propagation of a slow- S (or S2) mode in which the displacement vector is perpendicular to the fracture planes (Figure 3b). Note how much longer it takes for the S2 wavelet to propagate through the test sample than the S1 wavelet – a confirmation that S2 velocity is slower than S1 velocity. The positive-polarity end of the source is oriented in the direction indicated by the arrowhead on the source vector. For response A, the positive-polarity of the receiver is oriented the same as the source. For response C, the receiver has been rotated so that its positive-polarity end points in an opposing direction. Thus the polarity of wavelet B is opposite to the polarity of wavelet A. In actual seismic fieldwork with S-wave sources and receivers, the positive polarities of all receivers are oriented in the same direction across a data-acquisition template so that wavelet polarities are identical in all quadrants around a source station. At receiver orientations B and D, the receiver is orthogonal to the source vector, which produces zero-amplitude responses. The translation of these experimental results into exploration practice means that seismic prospecting across fracture prospects should involve the acquisition of S-wave data – and further, the data-acquisition geometry should allow S-wave velocity to be measured as a function of azimuth. When an azimuth direction is found in which S velocity across the depth interval of a fracture system has its maximum velocity, then the orientation direction of the dominant vertical fracture in that interval is defined as that maximum-velocity azimuth. S Waves Laboratory Experiments on Real-Earth Media Now we expand our insights into the behavior of seismic S waves as they propagate through a fractured interval, with the emphasis placed on laboratory data of real S waves propagating through fractured real-Earth media. The experimental data illustrated on Figure 4, taken from work published by Sondergeld and Rai, simulate the general case of S-wave illumination of a fracture system in which the illuminating source vector is polarized at an arbitrary angle Ф relative to aligned fractures. The test sample used to acquire the data was illustrated and discussed above. The wavefields that propagate through the medium are now a combination of S1 (fast-S) and S2 (slow-S) wavelets, and not S1-only or S2- only wavelets as were generated in the experimental data discussed above. Wavelets A, B, C and D are again the responses observed when the receiver is either parallel to or orthogonal to the illuminating source vector. The observed data contain both S1 and S2 arrivals. The length of the propagation path through the sample is such that the difference in S1 and S2 travel times causes the S1 and S2 wavelets to not overlap. In real seismic data, when a fracture interval is thin compared to a seismic wavelength and the difference in S1 and S2 travel times is not too large, the response will be a complicated waveform representing the sum of partially overlapping S1 and S2 wavelets. The wavelets at positions A’, B’, C’ and D’ illustrate important S-wave physics: ● Only a S1 mode propagates parallel to the fracture planes (responses A’ and C’). ● Only a S2 mode propagates perpendicular to the fracture planes (responses B’ and D’). The experiment documented as Figure 5 illustrates the results that should be observed when S-wave data are acquired across a fracture system as a 3-D seismic survey in which there is a full azimuth range between selected pairs of sources and receivers. In this test, the source and receiver are rotated in unison so that the positive-polarity ends of both source and receiver are always pointing in the same azimuth. This source-receiver geometry is what is accomplished during S-wave data processing when field data are converted from inline and crossline data-acquisition space to radial and transverse coordinate space that allows better recognition of S-wave modes. This type of source and receiver rotation is common practice among seismic data processors that have reasonable familiarity with S waves. The test data show convincing proof that only a fast-S mode propagates parallel to fractures, and only a slow-S mode propagates perpendicular to fractures. At all intermediate azimuths between these two directions, S-wave propagation involves a mixture of fast-S and slow-S wavefields. The objective of real-Earth fracture evaluation is to acquire seismic data in a way that allows source and receiver rotations to be done to create data similar to that shown on Figure 5. These rotated data are then searched to find the azimuth direction in which S-wave velocity is a maximum. That maximum-velocity azimuth defines the orientation of the set of vertical fractures that dominate the fracture population. P, S1, S2 Wave Laboratory Behavior: Cracked vs Uncracked Rock Experimental work done by Xu and King (1989) are presented as Figure 6. In this lab experiment, P, S1 (fast-S) and S2 (slow-S) modes propagate through a test sample before and after the sample was cracked to create a series of internal fracture planes. Wave transit times through the sample were measured to determine the effect of cracks on the velocity of each wave mode. For both the cracked and uncracked samples, transit time measurements were made for a series of confining pressure conditions varying from 1.4 MPa to almost 21 MPa. P-wave travel time behavior is described on the top panel of the figure; S1 and S2 travel times are summarized on panels b and c, respectively. On each data panel, the transit time for uncracked rock is marked as A. Point B indicates the travel time through the cracked sample. The travel times for P and S1 modes exhibit little pressure dependence over the applied pressure range for either cracked or uncracked media.
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